The invention relates to methods and apparatus which allow the measurement of extremely low rates of permeation of water, and more particularly to methods and apparatus for measuring an ultralow moisture permeation rate through objects such as plastic films and the like.
With the development of better and better barrier materials, generally from plastic films, it has become desirable to be able to precisely measure the rate of permeation through such barrier materials in order to properly evaluate them. As barrier materials have improved in their resistance to moisture permeation, it has become necessary to be able to accurately measure lower and lower rates of permeation.
Gas permeability measuring devices have generally been known in the art, and some of these have been directed to serve the garment industry where fabrics that are highly resistant to water permeation are often desired. However, more recently, with the development of LCD""s, LED""s and OLED""s, it has become important to develop barrier materials that have an extremely high resistance to moisture permeation and oxygen permeation. It has been shown scientifically that there is a relationship between the permeation of moisture and the permeation of oxygen through a barrier; the proportion is that, if there is a permeation of water equal to 1xc3x9710xe2x88x924 grams per unit time, for the same unit time, there will be a permeation of oxygen of about 1xc3x9710xe2x88x923. Accordingly by measuring the moisture permeation rate, an adequate assessment can be obtained for the resistance of a particular object, such as a barrier film, to the permeation of both moisture and oxygen.
Because many present day products have been found to be highly sensitive to oxygen and moisture, often resulting in significant deterioration of the product, there has been a recent emphasis on developing better barrier materials. Products in the electronics fields, such as OLED""s and LCD""s, and certain pharmaceuticals are among products for which it is most important to resist such deterioration. The barrier materials that have been developed to protect such materials generally include multilayer composites of plastic films and thin layer inorganic materials, and the search has gone on for providing increasingly better multilayer, thin film barrier materials for this purpose. For example U.S. Pat. No. 6,413,645 entitled xe2x80x9cUltrabarrier Substratesxe2x80x9d describes the problem and the search for more permeation-resistant materials.
In order to be able to effectively evaluate the performance of these new materials, adequate test equipment is required for detecting moisture permeation at these extremely low levels. Efforts have been made to use the amount of change in weight of a suitable desiccant in a closed container where the object closing the container has its opposite face exposed to a humid atmosphere as a measure of moisture permeation; however, the accuracy of such an apparatus has been frequently called into question. U.S. Pat. No. 4,663,969, issued May 12, 1987, discloses apparatus for testing water vapor transmission which employs a heated water bath; a solution containing a solute is employed along with an electric conductivity measuring device to measure the change in concentration, which will be indicative of moisture permeation. However, it is felt that such an apparatus is not suitable to measure extremely low rates of moisture permeation. U.S. Pat. No. 6,119,506 discloses an apparatus that is designed to allow measurement of mass transport. The flux of water vapor through a film or other object being measured is calculated by measuring results for exposure to a dry gas atmosphere, to a water-saturated atmosphere, and to atmospheres of different relative humidities; with a computer program being used to determine transmission rate for the object being tested. Humidity probes are used to provide outlet signals that are indicative of the water vapor concentration in nitrogen streams that are being caused to flow through a cell where such testing is occurring. In addition to being somewhat complicated, the apparatus is not felt to be well-suited to measuring extremely low moisture diffusion rates.
As a result, more accurate apparatus and methods have been sought for measurement of such ultralow permeation rates.
It has now been found that an apparatus for measuring ultralow water permeation through an object, such as a thin film, can be effectively created by utilizing tritiated water vapor (HTO). By suitably mounting the object to provide controlled access to opposite surfaces and by supplying tritiated water vapor to the upstream surface, vapor at the downstream surface can be collected and monitored to precisely determine even extremely low permeation rates through the object. The method particularly lends itself to execution by carefully controlling the humidity at the upstream surface and by creating a controlled flow of dry gas, such as nitrogen or methane, past the downstream surface. The flowing stream will collect the radioactive permeated HTO, and by causing it to flow past a radiation monitor, the moisture permeation rate can be quickly and accurately calculated.